For Halloween revelers, turning their eyes from blue to otherworldly with over-the-counter, decorative contact lenses may enhance holiday fun, but health experts warn these devices can lead to serious vision-threatening problems.

According to the Food and Drug Administration, all contact lenses are regulated medical devices that require a prescription and proper fitting by an eye-care professional. Even someone with perfect vision would still require an eye exam and a prescription in order to wear any kind of contacts, including cosmetic lenses.

"The buyer of these over-the-counter devices faces a huge risk," said Dr. Thomas Steinemann, professor of ophthalmology at MetroHealth Medical Center and Case Western Reserve University in Cleveland, and a clinical correspondent for the American Academy of Ophthalmology. "They can lead to blinding infection. Not pink eye but an ulcer on the cornea."

Some websites advertise decorative contacts as if they were fashion accessories or toys with fanciful, playful packaging and names like Dolly Eyes, to attract teens and young adults, especially girls. The problem is not that people use decorative, noncorrective lenses — sometimes called plano- or zero-powered lenses. But many users buy the devices without a prescription through unlicensed vendors on the Internet or at flea markets and specialty shops.

"All contact lenses, including lenses without corrective power, must receive marketing clearance or approval prior to being sold in the U.S.," said FDA spokeswoman Erica Jefferson. "Therefore, any over-the-counter sales are in violation regardless of whether the product has marketing clearance and approval,"

In 2005, a federal law was passed that classified all contact lenses as medical devices and restricted their distribution to licensed eye-care professionals, Jefferson said. Illegal sale of contacts can result in civil penalties of up to $11,000 per violation.

"If you get a contact lens without going through a doctor, you will have no education in care, cleaning and maintenance," said Hawke Yoon, a pediatric ophthalmologist at Children's Memorial Hospital in Chicago. "If it does not fit well, it could scratch the cornea."

"We see a few cases a year where young people misuse contact lenses and the result is diminished vision," said Richard Grostern, section director of Comprehensive Ophthalmology at Rush University Medical Center. "It's hard to quantify, but to wear these over-the-counter devices is putting yourself at risk."

"It's a pretty straightforward call," said Dr. Nicholas Volpe, chairman of the department of ophthalmology at Northwestern Memorial Hospital and Northwestern University Feinberg School of Medicine. "Don't wear a contact lens if you don't know how to properly take care of it and if it has not been properly fitted and prescribed by a physician."

Medical professionals concede that the risk of a worse-case scenario — blindness — is rare. But users of nonprescription contacts don't know if they might be that rare case.

"If used properly, these decorative contact lenses are not any more dangerous than prescription lenses," said Rachel Bishop, chief of Consult Services Section of The National Eye Institute. "But a street vendor selling a bunch of them does not know how these kids will use them or if they even know how. That's the danger."